This invention relates generally to optical equalization arrangements and, more particularly, to a method and apparatus for implementing a multi-channel optical equalizer for intersymbol interference mitigation.
Equalizers in the electrical domain to mitigate various impairments in digital optical communication systems are well known [see References 1-3 in Appendix]. An equalizer in the optical domain has been proposed but not demonstrated [4]. This proposed equalizer is a multistage lattice filter with many adjustable phase parameters, the parameters chosen by complicated optimization algorithms and compensates only one wavelength channel at a time. There have been demonstrations of single impairment optical compensators [4a], such as chromatic dispersion (CD) compensators [5-7] and polarization-mode dispersion (PMD) compensators [8], but these address only one source of impairment at a time.
What is desired is a simple multi-channel optical equalizer for intersymbol interference mitigation which can compensate many wavelength channels of a multiwavelength signal simultaneously and which requires few adjustable parameters.
In accordance with the present invention, we disclose a method and apparatus for implementing a less-complex single or multi-channel optical equalizer for intersymbol interference mitigation which can compensate many wavelength channels simultaneously and which requires fewer adjustable parameters. According to another aspect of our invention, when a semiconductor optical amplifier is used together with our optical equalizer, our optical equalizer can compensate for overshoots and signal transition degradations of the semiconductor optical amplifier. Our equalizer method and apparatus is extremely simple, having only two control signals (four in special cases), and yet can compensate many wavelength channels simultaneously. In one embodiment our equalizer includes a single Mach-Zehnder interferometer (MZI) with a free-spectral range of 50 GHz (an integer multiple of the channel spacing) and tunable couplers. In electrical equalizer terminology, our equalizer is a single-tap linear equalizer [3]. Our equalizer can dramatically improve the performance of non-return-to-zero (NRZ), carrier-suppressed return-to-zero (CSRZ), and possibly other transmission formats impaired by intersymbol interference. It can mitigate many impairments simultaneously, including those due to transmitter and/or receiver imperfections, filter narrowing, CD, and PMD. Our equalizer is especially dramatic in its improvement of the non-return-to-zero (NRZ) amplitude-shift keying (ASK) format, which is significant since this is the least expensive format to generate and detect.
More particularly, our optical signal equalizer simultaneously equalizes one or more received signals modulated at a preselected modulation bit rate in an optical system, the equalizer comprising
a first coupler with a variable coupling ratio for splitting the light into two or more portions;
a controllable interferometer unit having two or more arms, each arm receiving one portion, at least one arm having an additional delay which is equal to an integer
multiple of 1/xcex94f, where xcex94f is the channel spacing between adjacent wavelengths utilized in the optical system;
at least one arm having a controllable delay unit for adjusting the relative phase of the light passing through that arm;
and a second coupler for combining the signals from the arms.
In another embodiment, the optical signal equalizer for equalizing a received optical signal modulated at a preselected modulation bit rate is implemented using two or more equalizer units
According to our method of operating an optical equalizer of an optical system, we equalize a received optical signal modulated at a preselected modulation bit rate using the steps of:
splitting the light into two or more variable portions;
creating a differential delay between the two or more of the portions, which is approximately equal to an integer multiple of 1/xcex94f, where xcex94f is the channel spacing between adjacent wavelengths utilized in the optical system,
adjusting the phase of the light in one of the two or more portions,
combining the two or more portions into a single output, and
whereby the variable adjustments are made so as to mitigate intersymbol interference impairments in the received optical signal.